Dfa basics

Introduction to Finite
Automata
Languages
Deterministic Finite Automata
Representations of Automata
1

Alphabets
An alphabet is any finite set of
symbols.
Examples: ASCII, Unicode, {0,1} (binary
alphabet ), {a,b,c}.

2

Strings
The set of strings over an alphabet Σ is
the set of lists, each element of which is
a member of Σ.
Strings shown with no commas, e.g., abc.

Σ* denotes this set of strings.
ε stands for the empty string (string of
length 0).
3

Example: Strings
{0,1}* = {ε, 0, 1, 00, 01, 10, 11, 000,
001, . . . }
Subtlety: 0 as a string, 0 as a symbol
look the same.
Context determines the type.

4

Languages
A language is a subset of Σ* for some
alphabet Σ.
Example: The set of strings of 0’s and
1’s with no two consecutive 1’s.
L = {ε, 0, 1, 00, 01, 10, 000, 001, 010,
100, 101, 0000, 0001, 0010, 0100,
0101, 1000, 1001, 1010, . . . }
Hmm… 1 of length 0, 2 of length 1, 3, of length 2, 5 of length
3, 8 of length 4. I wonder how many of length 5?
5

Deterministic Finite Automata
A formalism for defining languages,
consisting of:
1.
2.
3.
4.

A finite set of states (Q, typically).
An input alphabet (Σ, typically).
A transition function (δ, typically).
A start state (q0, in Q, typically).

5. A set of final states (F ⊆ Q, typically).
“Final” and “accepting” are synonyms.
6

The Transition Function
Takes two arguments: a state and an
input symbol.
δ(q, a) = the state that the DFA goes to
when it is in state q and input a is
received.

7

Graph Representation of
DFA’s
Nodes = states.
Arcs represent transition function.
Arc from state p to state q labeled by all
those input symbols that have transitions
from p to q.

Arrow labeled “Start” to the start state.
Final states indicated by double circles.
8

Example: Graph of a DFA
Accepts all strings without two consecutive 1’s.
0
A

0,1
1

Start
Previous
string OK,
does not
end in 1.

B

1

C

0
Consecutive
Previous
String OK, 1’s have
ends in a been seen.
single 1.
9

Alternative Representation:
Transition Table
Final states
starred
Arrow for
start state

* A
* B
C

0

1

A
A
C

B
C
C

Columns =
input symbols

Rows = states

10

Extended Transition Function
We describe the effect of a string of
inputs on a DFA by extending δ to a
state and a string.
Induction on length of string.
Basis: δ(q, ε) = q
Induction: δ(q,wa) = δ(δ(q,w),a)
w is a string; a is an input symbol.
11

Extended δ: Intuition
Convention:
… w, x, y, x are strings.
a, b, c,… are single symbols.

Extended δ is computed for state q and
inputs a1a2…an by following a path in the
transition graph, starting at q and
selecting the arcs with labels a1, a2,…,an
in turn.
12

Example: Extended Delta
A
B
C

0
A
A
C

1
B
C
C

δ(B,011) = δ(δ(B,01),1) = δ(δ(δ(B,0),1),1) =
δ(δ(A,1),1) = δ(B,1) = C
13

Delta-hat
In book, the extended δ has a “hat” to
distinguish it from δ itself.
Not needed, because both agree when
the string is a single symbol.
˄
˄
δ(q, a) = δ(δ(q, ε), a) = δ(q, a)
Extended deltas
14

Language of a DFA
Automata of all kinds define languages.
If A is an automaton, L(A) is its
language.
For a DFA A, L(A) is the set of strings
labeling paths from the start state to a
final state.
Formally: L(A) = the set of strings w
such that δ(q0, w) is in F.
15

Example: String in a
Language
String 101 is in the language of the DFA below.
Start at A.
0
A
Start

0,1
1

B

1

C

0

16

Language
Follow arc labeled 1.
0
A
Start

0,1
1

B

1

C

0

17

Language
Then arc labeled 0 from current state B.
0
0,1
A
Start

1

B

1

C

0

18

Language

Finally arc labeled 1 from current state A. Result
is an accepting state, so 101 is in the language.
0
0,1
A
Start

1

B

1

C

0

19

Example – Concluded
The language of our example DFA is:
{w | w is in {0,1}* and w does not have
two consecutive 1’s}
Such that…

These conditions
about w are true.

Read a set former as
“The set of strings w…
20

Proofs of Set Equivalence
Often, we need to prove that two
descriptions of sets are in fact the same
set.
Here, one set is “the language of this
DFA,” and the other is “the set of strings
of 0’s and 1’s with no consecutive 1’s.”

21

Proofs – (2)
In general, to prove S=T, we need to
prove two parts: S ⊆ T and T ⊆ S.
That is:
1. If w is in S, then w is in T.
2. If w is in T, then w is in S.

As an example, let S = the language
of our running DFA, and T = “no
consecutive 1’s.”
22

Part 1: S ⊆ T

0

A 1 B 1C

To prove: if w is accepted by
Start 0
then w has no consecutive 1’s.
Proof is an induction on length of w.
Important trick: Expand the inductive
hypothesis to be more detailed than you
need.

23

0,1

The Inductive Hypothesis
1. If δ(A, w) = A, then w has no
consecutive 1’s and does not end in 1.
2. If δ(A, w) = B, then w has no
consecutive 1’s and ends in a single 1.
Basis: |w| = 0; i.e., w = ε.
(1) holds since ε has no 1’s at all.
(2) holds vacuously, since δ(A, ε) is not B.
“length of”

Important concept:
If the “if” part of “if..then” is false,
the statement is true.

24

Inductive Step

0

A 1 B 1C

0,1

Start 0

Assume (1) and (2) are true for strings
shorter than w, where |w| is at least 1.
Because w is not empty, we can write w
= xa, where a is the last symbol of w,
and x is the string that precedes.
IH is true for x.

25

Inductive Step – (2)

0

A 1 B 1C

0,1

Start 0

Need to prove (1) and (2) for w = xa.
(1) for w is: If δ(A, w) = A, then w has no
consecutive 1’s and does not end in 1.
Since δ(A, w) = A, δ(A, x) must be A or B,
and a must be 0 (look at the DFA).
By the IH, x has no 11’s.
Thus, w has no 11’s and does not end in 1.
26

Inductive Step – (3)

0

A 1 B 1C

0,1

Start 0

Now, prove (2) for w = xa: If δ(A, w) = B,
then w has no 11’s and ends in 1.
Since δ(A, w) = B, δ(A, x) must be A, and
a must be 1 (look at the DFA).
By the IH, x has no 11’s and does not end
in 1.
Thus, w has no 11’s and ends in 1.
27

Part 2: T ⊆ S

X

Now, we must prove: if w has no 11’s,
then w is accepted by 0
0,1
A 1 B 1C
Y
Start 0

Contrapositive : If w is not accepted by
0

A 1 B 1C
Start 0

0,1

then w has 11.

Key idea: contrapositive
of “if X then Y” is the
equivalent statement
“if not Y then not X.”
28

Using the Contrapositive

0

A 1 B 1C

0,1

Start 0

Every w gets the DFA to exactly one
state.
Simple inductive proof based on:
• Every state has exactly one transition on 1, one
transition on 0.

The only way w is not accepted is if it
gets to C.
29

Using the Contrapositive
– (2)

0

A 1 B 1C

0,1

Start 0

The only way to get to C [formally:
δ(A,w) = C] is if w = x1y, x gets to B,
and y is the tail of w that follows what
gets to C for the first time.
If δ(A,x) = B then surely x = z1 for some
z.
Thus, w = z11y and has 11.
30

Regular Languages
A language L is regular if it is the
language accepted by some DFA.
Note: the DFA must accept only the strings
in L, no others.

Some languages are not regular.
Intuitively, regular languages “cannot
count” to arbitrarily high integers.
31

Example: A Nonregular Language
L1 = {0n1n | n ≥ 1}
Note: ai is conventional for i a’s.
Thus, 04 = 0000, e.g.

Read: “The set of strings consisting of n
0’s followed by n 1’s, such that n is at
least 1.
Thus, L1 = {01, 0011, 000111,…}
32

Another Example
L2 = {w | w in {(, )}* and w is balanced }
Note: alphabet consists of the parenthesis
symbols ’(’ and ’)’.
Balanced parens are those that can appear
in an arithmetic expression.
• E.g.: (), ()(), (()), (()()),…

33

But Many Languages are
Regular
Regular Languages can be described in
many ways, e.g., regular expressions.
They appear in many contexts and
have many useful properties.
Example: the strings that represent
floating point numbers in your favorite
language is a regular language.
34

Example: A Regular
Language
L3 = { w | w in {0,1}* and w, viewed as a
binary integer is divisible by 23}
The DFA:
23 states, named 0, 1,…,22.
Correspond to the 23 remainders of an
integer divided by 23.
Start and only final state is 0.
35

Transitions of the DFA for L3
If string w represents integer i, then
assume δ(0, w) = i%23.
Then w0 represents integer 2i, so we
want δ(i%23, 0) = (2i)%23.
Similarly: w1 represents 2i+1, so we
want δ(i%23, 1) = (2i+1)%23.
Example: δ(15,0) = 30%23 = 7; δ(11,1)
Key idea: design a DFA
= 23%23 = 0.

by figuring out what
each state needs to
remember about the 36
past.

Another Example
L4 = { w | w in {0,1}* and w, viewed as the
reverse of a binary integer is divisible
by 23}
Example: 01110100 is in L4, because its
reverse, 00101110 is 46 in binary.
Hard to construct the DFA.
But theorem says the reverse of a
regular language is also regular.
37

Dfa basics

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Dfa basics